(1) Field of the Invention
The present invention relates to a digital double-side copier which can copy one or both sides of an original document onto one or both sides of a copy sheet.
(2) Description of Related Art
An electrophotographic copier essentially comprises an image reading unit with a document feeder, and an image forming unit with a copy sheet feeder. With this copier, a document is fed to an image reader by the document feeder, and an optical system reads an image on the document and converts it into image data, which are outputted into the image forming unit. Upon input of the image data, the image forming unit forms an electrostatic latent image on a photosensitive drum through exposure, thence a toner image with a supply of toner. Subsequently, the toner image is transferred and fixed onto a copy sheet fed from the copy sheet feeder through an image forming path.
To meet today's need, the electrophotographic copier was upgraded to enable double-side copying, and a digital-copier with the image reader is a good example. The image reading unit in this type of copier reads images on one or both sides of a document, and the image forming unit transfers images onto one or both sides of a copy sheet. Thus, users can select a desired side-mode among 4 combinations as set forth in Table 1.
TABLE 1 ______________________________________ Side-Mode Setting Reading Image Forming ______________________________________ Single/Single Single-side Single-side Single/Double Single-side Double-side Double/Single Double-side Single-side Double/Double Double-side Double-side ______________________________________
The image reading unit of the double-side copier additionally includes a document circulation path for feeding the document again into the image reader by reversing the side thereof. Likewise, the image forming unit thereof additionally includes a copy sheet circulation path besides the image forming path for feeding the copy sheet again into an image transfer station by reversing the side thereof.
To date, several types of double-side copiers are devised by exploiting the above construction. An example is disclosed in U.S. Pat. No. 4,499,500, wherein two memories are installed in the image forming apparatus: one for image data on the front side, and the other for the back side. Another example is disclosed in U.S. Pat. No. 4,568,169, wherein a transportation speed in the circulation path is varied in accordance with the number of copies or a copy sheet size to hold several copy sheets thereon while transporting them back to the image transfer station, enabling steadily image forming on one side per several copy sheets.
However, in the former, overall throughput is decreased considerably; for the image forming on the back side must be suspended until the copy sheet is fed into the image forming station again through the copy sheet circulation path. While in the latter, enhancing throughput eventuates in increasing manufacturing costs; for the larger a memory capacity becomes, the higher the manufacturing costs. More precisely, if the digital copier is to copy approximately fifty pages at a time, employing a memory with an adequate capacity increases manufacturing costs inhibitedly for commercialization.
Therefore, a double-side digital copier such that can optimize throughput without increasing manufacturing costs has been sought after.